I have a corner tank and use an 18" on the back half of the tank. Seems to work well enough to grow some basic or medium light demanding plants. In particular my red ludwigia is getting more red at the tops, so that's a good sign, I'd say.
Probably medium light directly under them and low light by the time you hit 4 to 6 inches out from the center.
Depending on your tank and setup, you may want two to get medium/high light.
I think they're great little fixtures, especially for the price.